This is the circuit of a simple burglar alarm that produces a loud beep when somebody crosses a protected area or door. The circuit is highly sensitive and can detect the shadow of the moving person from a distance of 1 meter. It does not require an aligned light beam to make the circuit standby. It is portable and can be places anywhere for monitoring.

Principle: LDR or Light Dependent Resistor is a specialized type of resistor made up of Cadmium Sulphide. Its resistance depends on the intensity of light falling on it. In dark, its resistance will be very high about 10 Meg ohms. When light falls on LDR its resistance reduces to a few ohms. Thus the resistance of LDR depends on the light intensity. So it is called as a photo sensor. Operational amplifiers are Integrated Circuits(IC) that are used in amplifier circuits. Op-Amps have two inputs. These are Inverting input(negative) and non inverting input(positive). The output of IC becomes high when the non inverting input gets more voltage that inverting input. When the condition reverses, output becomes low.

Shadow Alarm Circuit

Working Principle: Op Amp IC UA 741 is used as a voltage comparator. Its inverting input pin2 receives half supply voltage (4.5 volts) through the potential resistors R1 and R2. The non inverting input pin3 gets a variable voltage through LDR and VR. Normally when the LDR gets light, its resistance will be low and it conducts and provide a high voltage to the non inverting input of IC. This makes the output of IC high. The high output from IC is given to the base of T1 through a current limiting resistor R3. T1 is PNP transistor and it conducts only when its base becomes negative. Here normally the base of T1 will be high due to the high output from IC. So T1 remains off and Buzzer and LED connected to its emitter remains off. When a person passes in front of LDR, the shadow of the person reduces the resistance of LDR and the voltage at the non inverting input of IC decreases. This makes the output of IC low. Immediately T1 conducts activating Buzzer and LED indicating the entry of a person.

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About D.Mohankumar

D. Mohan Kumar M.Sc., M.Phil, is an Associate Professor from Trivandrum, Kerala, India. He has been guiding Post Graduate students in Animal Science research for the last 25 years in several premier institutions in Kerala, India. He is an avid electronics enthusiast and has been designing circuits for the past 20 years. In this period he has supported many electronics/electrical engineering students in their final year projects. He is a regular contributor to Electronics For You magazine which is South Asia’s largest circulating electronics magazine published from New Delhi.
He is also contributing articles in http://www.elprocus.com and http://www.engineersgarage.com/